This invention relates to a process for the manufacture of flat sheets or webs of natural fiber material which have been comminuted (broken down) into individual fibers such as cellulose fibers, in general, and more particularly to an improved method of making such sheets and webs which uses only a minimal amount of water.
In the conventional paper making processes large amounts of water are used for the suspension of cellulose fibers. Of this large amount of water required, only approximately 1% of the water ends up in the finished paper product. As a result, large quantities of water must be used and handled during the process. In particular, this large quantity of water must be removed from the paper as it is processed. This is generally done at least in part by using mechanical means, i.e., using squeezing or suction to remove the water, or through thermal means such as drying cylinders. Whichever type of removal is employed, a large amount of expensive equipment is needed, which equipment also requires great amounts of energy for driving, thereby substantially increasing the cost of making the paper. In this prior method of making paper, the floating of the cellulose material in the water leads to the formation of sheets through a purely mechanical process in that the relative movement of the individual fibers with respect to one another and their sliding together to form a netted sheet-like structure is enhanced. The adherance between fibers in the finished sheet however, is not only due to the purely mechanical netting but is also a result of chemical bonding forces of various kinds, the action of which is promoted or even made possible by the aqueous phase. A chemical part of the bonding causes the netted fibers to be fixed in their arrangement with respect to one another and to form a sheet structure engendering these common bonding forces. It can thus be seen that this process requires at least a minimum amount of water.
Other methods of forming sheet structures which do not use water have been developed. In general, these have substituted separate bonding means for the mechanical and chemical bonding forces developed from the water and the fibers themselves in the above described method. In one method disclosed in British Pat. No. 897,295, a sheet with bonded fibers is shown in which a substrate surface is coated with a bonding material and with the aid of an electrostatic field the fibers are then transferred to this bonding layer, after which the resulting sheet is separated from the substrate or forming surface. A similar process is described in Danish Pat. No. 120,930, in which process the electrostatic field is used to orient the fibers perpendicularly before their transferal to the bonding layer with the fibers then being blown over into the binding layer through the use of air jets. A plurality of layers of different orientations are positioned one above the other. A further process is shown in British Pat. No. 1,239,642. In this process which is used for bonding the fibers of a cellulose web, the cellulose is brought into contact with dimethylsulfoxide containing 3 to 30% by weight of NO.sub.2. This solution softens the cellulose fibers and results in a form of welding of the areas of contact of neighboring fibers without destroying the cohesiveness of the web. After a certain amount of time the solution is washed out and the web thereafter exhibits a substantially greater mechanical cohesiveness.
Although these various methods work quite well, it will be recognized that they require additional equipment, materials and so on. Thus, it can be seen that there is a need for an improved method which utilizes the inherent binding potential of the fibers themselves along with water without requiring the great amounts of water needed for the prior art process.